Microwave tube



Nov. 4, 1958 c, BIRDSALL ETAL 2,859,374

MICROWAVE TUBE Filed Dec. 18, 1952 5 Sheets-Sheet 1 I INVENTOR. 624M255 /(.5/ifif441., y 043M 4. Mark/Mr,

5 c. K. BIRDSALL ETAL 2,859,374

MICROWAVE TUBE Filed Dec. 18, 1952 5 Sheets-Sheet 5 IN V EN TORS.

4,1958 K; BIIYRDSALL ET AL 2,859,374

MICROWAVE TUBE 5 Sheets-Sheet 4 Filed Dec. 18, 1952 C. K. BIRDSALL ETAL MICROWAVE TUBE Nov. 4, 1958 5 Sheets-Sheet 5 Filed Dec. 18, 1952 INVENTORV. 6/1/01: x4 5/101414, y alum ,4 ha r/0w, 4%J 2a a u n n Unite Sttes Patent MICROWAVE TUBE Charles K. Birdsall, Los Angeles, and Dean A. Watkins,

Pacific Palisades, Calif., assignors, by mesne assignments, to Hughes Aircraft Company, a corporation of Delaware Application December 18, 1952, Serial No. 326,632

8 Claims. (Cl. 315-3.6)

This invention relates to wave-type electron stream amplifier tubes and, more particularly, to a method of and means for obtaining signal amplification or attenuation by utilizing an electron stream.

In the conventional operation of a traveling-wave tube, an electromagnetic wave having a phase Velocity slightly less than that of an electron stream is directed along the path of the electron stream. The effect of this Wave on the electron stream is to cause the electrons of the stream to be attracted to certain regions of the'wave, thereby forming what may be called space charge waves or elec tron bunching within the electron stream. Inasmuch as the electrons of the stream have a slightly higher velocity than the wave, a major portion of the electrons within each bunch is decelerated by the electric field of the wave, resulting in a transfer of energy from the electron stream to the wave.

The bunching of the electrons of the stream is a dynamic process, that is, because of the finite mass attributable to an electron, the electrons may be momentarily bunched tightly, but such tight bunching cannot be maintained intact. The mutual repelling forces between the electrons within a .bunch, cause the electrons to expand or,

debunch. The electric field of the wave subsequently causes the electrons of the stream to bunch again, thus repeating the bunching process.

Once bunching of the electron stream is initiated in accordance with a signal tobe amplified, the tightness of bunching within the electron stream is representative of the signal energy being propagated by the stream. This is true, irrespective of whether or not the electron stream is flowing in the presence of an electromagnetic wave. It is to be noted that both signal energy or noise may be represented by perturbations of the electrons of the stream, that is, by disturbances or motion of the electrons other than their regular motion along the path of the stream. More specifically, in the case of signal energy, the perturbations of the electrons at any point along the stream are uniform and periodic variations of the electron motions, and in the case of noise, the perturbations are random. The rate at which the electrons bunch and debunch is referred to as the plasma frequency, and the distance along the direction of flow of the electron stream required for a complete cycle of bunching is referred to as a plasma wavelength. In this respect it is to be noted that the stream electrons go through two phases of maximum bunching for each complete cycle of bunching. Also, the plasma frequency is a function of the average charge density of the electron stream and represents the hindrance to bunching and the aid to debunching.

-In an electron stream tube, a factor which modifies the plasma frequency and hence the resistance to bunching is generally referred to as a reduction factor. The reduction factor is a function of the circuit or structure surrounding the electron stream and more specifically, a function of the displacement current in the stream, the displacement current outside of the stream, and the axial ice component of conduction current in the circuit congeneral, the reduction factor is a function of the ratio of.

the cross sectional dimensions of the circuit to the diameter of the electron stream through which the electron stream is flowing and of the potential through which the electrons of the stream have been accelerated.

The present inventionis directed towards a wave-type tube capable of amplifying or attenuating a microwave signal. This is accomplished by projecting an electron stream modulated by the signal through sections onequarter plasma wavelength long having alternately large and small reduction factors. nation available is dependent upon the ratio of the reduc-' tion factors of successive sections. Various values of reduction factor may be obtained by using conducting cylindrical sections of different diameter, an inductive wall section, or a helix having a pitch smaller than that required for synchronism with the space charge wave;

propogated by the electron stream. The operation of the tube of the present invention may best be explained by an analogy to the action of a pendulum, the pivot of which moves uniformly on a trackthrough regions having different gravitational forces. The pendulum is allowed to swing up in a region having a small gravitational force and swing down in a region having a large gravitational force. In such a case, the amplitude of the swing of the pendulum will obviously increase with time. Should the pendulum swing up in. a region having a large gravitational force, the opposit result, namely, damping, would be obtained. As previously mentioned, the reduction factor of a section is representative ofits resistance to bunching of the stream electrons.

electrons change. This process is similar to the change. from kinetic to potential energy of the pendulum, that is,; a condition whereby the electrons of the stream are being accelerated toward each other corresponds to a state of high kinetic energy, and a condition whereby the electrons are tightly bunched, corresponds to a state of high potential energy. The degree of bunching, of course, is representative of the degree of modulation and, hence, the signal amplitude. It is to be noted that the average kinetic energy of the stream electrons is not considered in the foregoing statements. Inasmuch as a complete cycle of energy transfer during the bunching process in the electron stream occurs at the plasma frequency, acomplete period of the pendulum will correspond to the time required for a plasma cycle. Hence, each upswing or downswingof the pendulum can be said to be representative of one-quarter of a plasma wavelength. 1 Either gain or attenuation may then be obtained from an electron stream type tube embodying the foregoing principles of operation. Whether or not gain or attenuation is obtained, depends upon the orientation of the bunching; cycle in the electron stream with respect to' the sections 1 The amplification or atten-.'

During the various stages of the. bunching process, the forms of available energy of the.

for example, a klystron. In general, the basic, cause. of noise in the output of a traveling-wave tube is due to the random velocities of the stream electrons. One embodiment of the. present. invention alleviates this condition. by'attenuating the noise prior to the application of signal modulation of the electron stream.

One wave-type tube that maybe considered as being of the same general class as a tube embodying the disclosed amplifier is known as a velocity-jump amplifier. In a tube of this type, an electron stream is modulated and projected through sections having different potentials, thereby causing the velocity of the electrons to change suddenly. The gain obtainable from an amplifier of this type is a function of the ratio of the potentials, applied to successive sections of the tube and of the ratio of the reduction factors. In this case, however, the reduction factors are changed, due mainly to a change in the velocity of the electron stream and in general not due to the change of the circuitry or structure surrounding the stream. In fact, in the velocity-jump tube, the ratio of reduction factors generally serves to decrease the amplification or attenuation available, as the case may be.

An advantage of the present invention over tubes of the foregoing and other types requiring different potentials along the path of the electron stream is that magnetically focused or Brillouin flow of the electrons of the stream can be'used. As is commonly known, Bril louin flow is initiated by launching the electron stream prior to the beginning of the axial magnetic field. Due to the radial components of the magnetic lines of force present at the beginning of the axial portion of the magnetic field, the electrons follow a spiral path in such a manner that the outward space charge force and the centrifugal force acting on the electrons are counteracted by an inward force produced by the circular motion of the electrons through the axial magnetic field, thereby focusing the electron stream. Since the centrifugal space charge force is a function of the current, velocity, and diameter of the stream and the centripetal forces are functions of the rotational velocity of the electron as brought about by the magnetic field, there exists a critical relation among the current, velocity, and diameter of the stream and the magnetic field to produce a stream with a smooth boundary. Thus, once Brillouin flow is initiated, it is necessary that a substantially constant potential be maintained near the path of the electron stream so as not to disturb the dynamic balance through which focusing is achieved; Were the potential to vary appreciably, the stream envelope would be distorted greatly from smooth flow. Hence, Brillouin flow may be used in tubes of the present invention, whereas it could not be Used in tubes of the velocity-jump type, thus enabling a substantial reduction in the magnetic flux required for the axial magnetic field. The axial magnetic field required for Brillouin fiow is of the order of 50 percent less than that required for a magnetically confined flow of electrons as is now widely used.

Another advantage of the present invention over conventional traveling-wave tubes is that the internal feedback causing the tube to oscillate is essentially eliminated.v In a conventional traveling-wave tube, an electron stream flows contiguous to a circuit capable of propagating an electromagnetic wave at about the velocity of the stream. In case of a mismatch or other disturbance at the output, a portion of the electromagnetic wave may be reflected towards the input. Inasmuch as the circuit is. birdirectional, the, reflected wave may reach the im put at asuflicient amplitude so that subsequentamplification of the wave in passing through the tube results in a signal of increased amplitude appearing at the output, causing the tube to oscillate. In the present invention, thesections of different reduction factors, in the absence. of the; electron stream, will generally attenuate very highly any electromagnetic wave in the range, of

frequencies for which the tube will amplify. Since the 4 signal, which is propagated by the electron stream in the form of a space charge wave, can only proceed in a direction along the flow of the electron stream, internal feedback is inherently absent.

In addition to the foregoing advantages, a tube incorporating the disclosed method of amplification may provide gain comparable to that obtainable from conventional traveling-wave tubes over an equivalent range of frequencies. Also, there is no electronic lens action by static electric fields on the electron stream in the disclosed wave-type tube, as there is in the velocity-jump tube between sections to which different potentials are applied. This lens action is not only undesirable, but also unavoidable in tubes of the velocity-jump type. Furthermore, there is no ion trapping in a tube incorporating the present invention, whereas this is not true in regard to the low voltage sections of the velocity-jump tube. By ion trapping" is meant the accumulation of positively charged particles in a region of low potential. through which the electron stream, flows.

creases the noise power in the electron stream, which is undesirable.

It is therefore an object of this invention to provide a novel method of amplification for signals comprising frequencies in the microwave spectrum,

Another object of this invention is to provide a method of attenuating the noise in the electron stream of a tube.

An additional object of this invention is to provide a wave-type tube incorporating one quarter plasma wavelength sections having diiferent reduction factors for amplifying the modulations of an electron stream.

A further object of this invention is to provide an electron tube incorporating sections having different reduction factors for reducing the random noise in an electron stream.

A still further object of this invention is to provide a wave-type tube incorporating drift tube sections having a low reduction factor interposed between helical sectionshaving a high reduction factor for amplifying the modulations of an electron stream.

Still another object of this invention is to provide a wave-type tube including drift tube sections having a low reduction factor interposed between inductive wall sections having a high reduction factor for amplifying the modulations of an electron stream.

The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description considered in connection with the accompanying drawings in which several embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings, are for the purpose of illustration and description only, and are not intended as a definition of the limits Qf the invention.

Figs, 1, 6, 7 and 8 are diagrammatic sectional views of different embodiments of the present invention together with, associated circuits;

Figs. 2 and 3 are views in perspective illustrating the flow of electrons in the vicinity of conducting surfaces; and

Figs. 4 and 5 are graphs illustrating the variations of the reduction; factor as a function of the characteristics of' the electron stream and the signal frequency for the configurations illustrated in Figs. 2 and 3, respectively.

Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in Fig. l anembodirnent of atraveling-wave tube incorporating the present invention, Fig. 1 illustrates an envelope 10, which provides the necessary evacuated chamber for the traveling-wave tube, and consists of an elongated cyl- Interaction. between the trapped ions and electrons in the stream inassasrs in the center section as viewed in the drawing. Within the enlarged portion at the left extremity of envelope 10, there is an electron gun 12 for launching an electron stream comprising a cathode 14 with a heater 16, a focusing electrode 18 and an anode 20. Heater 16 is connected across a source of potential, such as a battery 22, the negative terminal of which is connected to cathode 14 which may be grounded as shown. Focusing electrode 18 provides a conductive conical surface disposed concentrically about the electron stream which requires zero volts with respect to cathode 14 in order to produce focusing of the electron stream and hence may be connected directly to the cathode.

- Anode 20 is connected to an adjustable contact arm 24 of a potentiometer 26 which is connected across a source of potential 28. Potentiometer 26 is utilized to I impress a variable positive potential on anode 20 which functions as a control element for determining the current in the electron stream. A potential of +500 volts with respect to ground is representative of the potential normally applied to anode 20.

A solenoid 76 is axially positioned symmetrically about the complete length of envelope 10. An appropriate' direct current is maintained in solenoid 76 by means of a potential source, such as a battery 78, so as to produce a magnetic field running axially along the length of the traveling-wave tube In the case that a constrained electron flow i used, an appropriate intensity for this magnetic field will be of the order of from 500 to 1000 gauss, whereas when Brillouin flow is used, an intensity of from 250 to 500 gauss is suflicient. The purpose of this magnetic field, of course, is to keep the electron stream focused or constrained while directing it along a path on the longitudinal axis of envelope 10. Solenoid 76, as illustrated in Fig. 1, is in an appropriate position for initiating Brillouin flow in the manner explained hereinabove.

Proceeding along the direction of flow of the electron stream there are positioned successively a matching ferrule 30 connected by a lead 32 to a conductive input helix 34, a reduction factor section 36, a conductive output helix 38 connected by a lead 40 to a matching ferrule 42, and a collector 44.

Helices 34, 38 which serve to modulate and demodulate the electron stream, are terminated by matching ferrules 30, 42 respectively, and usually have an inner diameter substantially equal to'the inner diameter of ferrules 30, 42 so that the stream electrons can be made to pass as closely to the helices as possible without being intercepted by the latter. A material such as tungsten is suitable for making the helical sections, the principal requirement being that they retain their form, especially with respect to their pitch and diameter. The potential applied to helices 34, 38 and matching ferrules 30, 42 produces the static electric field throughout the efi-ective length of these elements and, hence, determines the average velocity of the electron stream within the helices and matching ferrules. The velocity of the electron stream in passing through helices 34, 48 should be about the same as the phase. velocity'of an electromagnetic wave propagated by the helix in these regions so that the stream velocity must be variable, in order to obtain optimum modulation and demodulation of the electron stream. A potential of 1000 volts with respect to ground is representative of the magnitude of a potential suitable for thispurpose.

The application of an adjustablepotential to matching ferrules 30, 42 and to helices 34, 38, respectively, may be accomplished as shown by connecting ferrules 30, 42 to adjustable taps 46, 47 of a potentiometer 48 which is connected across a source of potential 50.

Resistive coatings 52, 54 which may be of carbon b1ack,.are applied on the outside of envelope about the last few turns of helices 34, 38 respectively, adjacent inder' with enlarged portions at the left extremity and to reduction factor section 36 for the purpose of terminating the electromagnetic portion of the traveling wave propagated by the helices. It is to be noted that there are numerous other means of terminating an electromagnetic wave, the described termination means being merely for the purpose of illustration.

Reduction factor section 36, located within the enlarged center portion of envelope 10 between helices 34 and 38 consists, for example, of a succession of concentric cylindrical sections 56, 58, 60 and 62 having alternately large and small diameters which are axially aligned with the electron stream. The cylindrical section 56, positioned nearest to electron gun 12, has an inner diameter that is large with respect to the diameter of the electron stream and, hence, has a correspondingly large reduction factor. The length of section 56 is equivalent to one-quarter of a plasma wavelength of the space charge wave propagated by the electron stream as measured in section 56.

Cylindrical section 58, positioned adjacent to section 56, has an inner diameter substantially equal to that of matching ferrules 30, 42' and, hence, has a correspondingly small reduction factor. The length of sec tion 58 is also equivalent to one-quarter of a plasma wavelength of the space charge wave propagated by the electron stream as measured in section 58. It is to be noted that a plasma wavelength in a region having a large reduction factor is shorter than in regions of small reduction factor, hence, section 56 is correspondingly shorter than section 58.

As was previously mentioned, the ratio of the reduction factors for successive cylindrical sections is a function of the gain obtainable from the tube. Cylindrical sections 60, 62 are similar to sections 56, 58 respectively. Flat circular plates 64, 66, 68, positioned perpendicular to the direction of flow of the electron stream, connect cylindrical sections 56, 58, 60, 62 together so as to make the enclosure about the electron stream provided by reduction factor section 36 continuous and conductive as shown in the drawing. Also, because the diameter of sections 58, 62 is too small compared to a wavelength at the frequency of amplification, any electromagnetic wave propagating through these sections in the absence of the electron stream will be very highly attenuated. Therefore, since the electron stream flows from the input to the output, there is no possibility of a feedback signal being propagated towards the input by the reduction factor section 36.

Inasmuch as the potential of reduction factor section 36 determines the nature of the static electric potential of the field which predominates the region traversed by the electron stream, a potential of the same orderof magnitude as that applied to helices 34, 38 is applied to reduction factor section 36 so as to cause a minimum disturbance in the flow of the electron stream, particularly in the case where Brillouin flow is used. Since a plasma wavelength is a function of the velocity of the electron stream, it is desirable that the potential applied to reduction factor section 36 be made adjustable for optimum gain or attenuation. This is accomplished by a connection from reduction factor section 36 to an adjustable contact arm 72 of a potentiometer 74 which is connected across potential source 50. A potential of the order of 1000 volts with respect to ground is representative of the magnitude of potentialsgenerally used for this purpose.

The collector 44 constitutes, the termination of the path along which the stream electrons are directed and hence is disposed to intercept the entire stream. .A suitable potential that is slightly positive with respect to the potential of ferrule 42 is applied to collector 44 so as to prevent secondary electrons that may be emit ted from the surface thereof from going back towards the electron gun 12. This is accomplished by connecting collector 44 to the positive terminal of potential gasses 7 source 50. A potential of the order of '30 volts positive with respect to the potential applied to ferrule 42 is representative of potentials generally used for this purpose.

As previously mentioned, helices 34, 38 are connected, respectively, to ferrules 3t}, 42 by leads 32, 40 so that the helices and ferrule are maintained at the same potential.

An "input waveguide section 88 and an output waveguide section 82 enclose the envelope 10 coextensive with leads 32, 40, respectively. Leads 32, 46 are disposed parallel to the respective electric fields developed within the waveguide sections at a distance approximately onequarter of a guide wavelength at the operating frequency from respective short-circuit terminations of the waveguide sections. Disposition of leads 32, 40 within input waveguide section 80 and output waveguide section 82, respectively, in this manner provides optimum transfer of energy between the waveguide sections and the input and output helices. Input and output waveguide sections 86, 82 have collars 81, 83 respectively which are positioned concentrically about ferrules 3t 42 respectively, and have a length equivalent to one-quarter of a wavelength Within the waveguide so as to produce apparent shorting planes at the inner surface of the waveguide sections.

In the operation of the amplifier tube of Fig. 1, an input microwave signal is impressed through input waveguide 80 to induce a signal potential on the lead 32 connecting matching ferrule 30 to input helix 34, thereby exciting a traveling wave in the helix. As is conventional, the 'axial phase velocity of the traveling wave through the helix is slightly less than the velocity of the electron stream in order to enable the wave to initiate an increasing space charge wave within the electron stream, thereby modulating the stream in accordance with the signal.

The traveling wave energy upon reaching the end of helix 34 consists of an electromagnetic portion propagated by the helix and a space charge portion propagated in the form of electron bunching by the electron stream. The diameters of cylindrical sections 58, 62 of reduction factor section 36 are sufliciently small in comparison to a wave length at the frequency of the traveling Wave so that the traveling Wave will be very highly attenuated in the absence of the stream electrons. Hence, when the traveling Wave energy reaches the end of helix 34 adjacent to reduction factor section 36, resistive coating 52 provides impedance matching for the purpose of terminating the electromagnetic portion of the traveling wave energy.

Thus, the signal energy, upon entering reduction factor section 36, is in the form of bunching of the electrons of the stream. As previously mentioned, cylindrical section 56 has a large reduction factor and, hence, offers large resistance to bunching. In order to obtain optimum amplification, the leading edge of section 56 is located'at. a point along the electron stream where there is maximum electron bunchingor electron density and terminates at the next successive point along the stream where the electrons have maximum bunching velocity. This distance constitutes one-quarter of a plasma wavelength. The net physical etfect on the electron stream, while flowing in a region having a high reduction factor, such as that provided by cylindrical section 56, is to aid the debunching process, that is, to aid in converting the potential energy of the tightly bunched electrons into the form of kinetic energy of the stream electrons.

Immediately following cylindrical section 56 there is located cylindrical section 58 which provides a region where the resistance to bunching is low. The electrons of the stream enter section 58 at a point of maximum bunching velocity and, hence, immediately commence to bunch, but inasmuch as they are now flowing in a region having low resistance to .bunching, the extent .of the bunching at the end of section 58 is considerably more and velocity in a plasma wavelength available from atraveling-wave tube of the type described having four one-quarter plasma wavelength sections, two sections of which have a reduction factor R spaced alternately with two sections of reduction factor R with R preceding R: with respect to the direction of flow of the electron stream, is

. z a an 5. 1 t 2] 1 R1 (1) Gain i 40 log 10 & decibels per plasma wavelength (2) wherein i i and v v; are the signal frequency perturbation of the average current density and velocity after leaving and before entering the reduction factor sections, respectively.

The amplified space charge wave propagated through reduction factor section 36 emerges at the far end where it induces a voltage on the helix 38, thereby transforming a portion of the signal energy in the electron stream into the form of an electromagnetic growing wave on the output helix 38. The stream electrons continue to impart energy to the growing wave on the helix as they proceed toward ferrule 42 until a greater portion of the signal energy in the stream is transferred to the wave propagated by the helix. After the transfer of the signal energy from the stream to the helix, the electrons are intercepted and their energy dissipated by collector electrode 44. Inasmuch as the electric field generated by the electromagnetic Wave on lead 40 connected to output helix 38 is parallel with the electric field of the fundamental mode desired to be excited in the waveguide, hence, the amplified signal energy is transferred from output helix 38 to the output Waveguide 82.

For convenience, a few curves are shown in Figs. 4 and 5 wherein the reduction factor, R, has been calculated for plane and axially symmetric fields for an electron stream confined to flow between perfectly conducting surfaces. More specifically, Fig. 2 shows an arrangement providing axial symmetry between an electron stream flowing in a region 84 of circular cross section and a concentric metal cylinder 85. Fig. 3 illustrates a tube structure providing plane symmetry for a sheet of electrons flowing in a region 86 between plane metal walls 87 and 88. ln Figs. 2 and 3, a represents the perpendicular distance from the line of symmetry of the electron stream and surrounding circuit to an exposed metallic surface and b represents the perpendicular distance from the line of symmetry to the outer boundary of the region of electron flow. Similar curves may be calculated for Brillouin flow.

Fig. 4 then illustrates the variation of the plasma frequency reduction factor R as a function of tab for axial symmetry Fig. 2 for ratios of by curves 90, 92 and 94, respectively, wherein m is the angular frequency of the signal impressed on the electron stream and a is the velocity of the stream.

Fig. 5 illustrates the variation of the plasma frequency reduction factor R as a function ofi "o for plane symmetry, Fig. 3 for ratios of 2, and 1 by curves 96, 98 and 100 respectively, wherein in and u are defined as hereinbefore. Optimum ratios of reduction factor for tubes incorporating the disclosed method of amplification can be obtained from curves of the type illustrated in Figs. 4, 5 by choosing an appropriate cross sectional dimension, b, of the electron stream and electron stream velocity, u for a particular angular frequency, or, desired to be amplified. In connection with the foregoing discussion, the curves illustrated in Figs. 4, 5 are, in each instance, for the fundamental mode of propagation of the space charge wave by the electron stream. Differences in reduction factors for higher order modes of propagation can also be obtained in a similar manner.

From the foregoing curves illustrated in Figs. 4, 5, the characteristics of the electron stream and the angular frequency of the signal impressed on the electron stream, one may ascertain the reduction factorprevailing along the path of the electron stream. A plasma wavelength A is a plasma wavelength,

R is the reduction factor which may be determined by the curves of Figs. 4 and 5,

to is the plasma angular frequency,

' u is the velocity of the electron stream, 7

e is the charge of an electron, I is the current density of the electron stream, m is the mass of an electron, and s0 is the absolute dielectric constant of free space.

Inspection of Equation 4 reveals that the ratio,

remains constant once the characteristics of the electron stream have been determined. Hence it can be seen' from Equation 3 that an electron stream of predetermined characteristics has a plasma wavelength, A which is-a' function of the reduction factor, R, prevailing along thepath' of the electron stream. From Figs-4, it was shown that the reduction factor, R, is a function of the. angular frequency a: of the signal impressed on the electron stream, of the velocity ac and of the cross, sectional dimension b of the electron stream. More generally, the angular signal fiequency, w, may be defined as the periodicity of the perturbations of the electrons of the stream. From the foregoing relations then, it can be'noted that an electron stream of predetermined characteristics has a plasma wavelength, M, which is a functioniof the reduction factor, R, prevailing along the path of" the" electron stream and the periodicityof the'perturbations of the electrons of the stream.

1 Another embodiment of a traveling-wave tube, in accordance with the invention, is illustrated in Fig. 6. In this embodiment, the diameter of the amplifying section may be considerably reduced by replacing the large re duction factor cylinders 56, 60 shown in Fig. l by helices 10 having a specified pitch relative to the pitch of the input and output helices 34, 38.

Referring to Fig. 6, an envelope 102 provides the necessary evacuated chamber for the traveling-wave tube and consists of an elongated cylinder with an enlarged portion at the left extremity as viewed in the drawing. Within the enlarged portion, there is the electron gun 12 for developing an electron stream connected as shown in Fig. l.

Proceeding along the direction of flow of the electron stream, there are positioned successively, the matching ferrule 30 connected by the lead 32 to the conductive input helix 34, a reduction factor section 103, which differs from reduction factor section 36 of Fig. 1, the conductive output helix 38 connected by the lead 40 to the matching ferrule 42, and the collector 44.

Reduction factor section 103 comprises, in a direction along the electron stream, a conducting tubular element 104 positioned adjacent to input helix 34, a high reduc tion factor helix 106, a conducting tubular element 108, and a high reduction factor helix 110. Tubular elements 104, 108 are in electrical contact with helices 106, 110 so that the entire reduction factor section 103 is at a common potential. A suitable potential is applied to section 103 by a connection to adjustable contact arm 72 of potentiometer 74.

Conducting tubular elements 104, 108 may be fabricated from any non-magnetic conducting material and have a diameter just suflicient to accommodate the electron stream so as to have a small reduction factor. an electron stream having essentially uniform velocity throughout the tube, helices 106, 110 have a substantially larger number of turns per unit length as do the input and output helices 34, 38 so as not to interact in a traveling wave fashion, but rather to have a reduction factor substantially the same as a conducting cylinder oflarge diameter relative to the stream diameter. The reduction factors applicable to a helix of this type are substantially the same as for an electron stream flowing concentrically in a conducting metal cylinder wherein the ratio of the cylinder diameter to the stream diameter is infinite. These values of reduction factor R are represented by curve of Fig. 4. A material such as tungsten is suit-v able for making helices 106, 110. In addition, tubular elements 104, 108 and helices 106, are, of course, each one-quarter plasma wave-length long.

The remaining apparatus and associated circuits for the embodiment shown in Fig. 6 is the same, respectively, as was shown and described for the tube illustrated in Fig. l. The manner of operation of the traveling-wave tube illustrated in Fig. 6 is also similar to the operation described for the tube illustrated in Fig. 1 obviating its repetition.

An additional embodiment of a traveling-wave tube of the invention is shown in Fig. 7 whereby the foregoing techniques are utilized to attenuate noise in a predetermined portion of the frequency spectrum in an electron stream prior to modulation of the stream or utilization of it for amplification purposes. As is generally known, the random velocities of the electrons of the stream with respect to their average velocity constitutes noise which generally limits the applications of stream type tubes and other electron tubes such as, for example, klystrons. In the present embodiment, two elements, one having a large reduction factor and the other a small reduction factor, are utilized to attenuate this noise in the electron stream before it is used for amplification purposes.

In accordance with the present invention, determination as to Whether a large or small reduction factor section is to be placed first with respect to the direction of flow of the electron stream, depends on the location of current and voltage maximums within the electron stream. In order to obtain optimum amplification, a first section having the larger reduction factor should commence at a point along the electron stream where there is a cur- For Current maximum or velocity minimum. Gain of signal current.

First section commensing at First section having the larger reduction factor.

First section having the smaller reduction factor.

Velocity maximum or current minimum. Attenuation of signal or noise velocity.

Attenuation of signal Gain of signal elocity. or noise currentv From the foregoing table, it will be evident that if the reduction factor sections are positioned so as to attenuate, for example, noise current along the electron stream, the same sections will amplify noise velocity. Because of this phenomenon, it is preferable to attenuate only over a small number of sections so as not to unduly amplify noise, represented either by electron current or velocity, not attenuated. After the noise in the electron stream is attenuated, the electron stream may be used in any conventional manner for the purpose of amplifying microwave signals. Alternatively, it may be desirable to attenuate electron noise current which, of course, would cause amplification of noise velocity. The output voltage resulting from the noise velocity can be effectively eliminated by using modulation and demodulation means that are very insensitive to velocity variations such as, for example, resonant cavities.

An embodiment of the type described is illustrated in Fig. 7, wherein a reduction factor section 115 comprising elements 116 and 117 in electrical contact with each other are disposed along the path of the electron stream in the direction of its flow in the order named prior to its being used for amplification purposes. Element 117 is tubular in shape and has a low reduction factor, whereas element 116 is a helix having a smaller pitch than required for synchronism so as to provide a large reduction factor. Both elements 117 and 116 are one-quarter plasma wavelength long.

Reduction factor section 115 is maintained at a suitable positive potential with respect to ground so as not to impede the flow of electrons. This is accomplished by a connection from reduction factor section 115 to an adjustable arm 119 of a potentiometer 120, which is connected across the potential source 28.

A conventional amplifier means such as, for example, a Wave-type or klystron amplifier, is positioned along the electron stream after reduction factor section 115 with respect to the electron flow. For the purposes of illustration, a traveling-wave amplifier means 124 is shown which may comprise a matching ferrule 126 connected by a lead 127 to a helix 128 which, in turn, is connected by a lead 129 to a matching ferrule 130 positioned concentrically about the electron stream along its direction of flow in the order named. In addition, a waveguide input means 131 and a waveguide output means 132 enable the electron stream to be modulated and demodulated, respectively. Ferrules 126, 130 and helix 128 are maintained at an appropriate positive potential with respect to ground by a connection to an adjustable contact arm 46 of a potentiometer 48 which is connected across source of potential 51 the negative terminal of which is connected to ground.

The electron gun 12, solenoid 76, and collector 44, together with associated circuits are as described in connection with the embodiment of the present invention illustrated in Fig. 1.

The operation of the tube as illustrated and described is. conventional except for the reduction factor section 115. Inasmuch as the frequency range, throughout which his desired to attenuate noise, may shift in accordance with the frequencies to be amplified, the potential applied tothe reduction factor section 115, which determines'the averageveloeity cf-the electrons of the stream through section thereby controlling its electrical length, is ad-- justed so that elements 116, 117 are each one-quarter plasma wavelength long corresponding to the frequency at the center of the operating frequency range. The position of reduction factor section 115 along the electron stream may be initially adjusted so as to give minimum noise at the output waveguide section 132.

Still another embodiment of the present invention is shown in Fig. 8 wherein inductive walls are used to provide large reductionfactors which may be substantially greater than unity. Inasmuch as the value of reduction factors available from conductive cylindrical sections or helical sections cannot exceed unity, as is evident from the prior discussion of Figs. 4, S, the gain available from a tube embodying a reduction factor section incorporating inductive walls to provide the necessary large reduction factor regions may be considerably greater than the gain available from tubes operating with reduction fac-' tors less than unity since gain, as previously mentioned, is proportional to theratio of the reduction factors of sod cessive one-quarter plasma wavelength sections.

Inductive walls may be provided by several diflferent means, some of which are described in copending applications for patent entitled, Electron Stream Amplifier Tube, by Andrew V. Haefl, Serial No. 282,000, and filed on April 12, 1952, now U. S. Patent No. 2740917, and Resistive-Inductive Wall Amplifier Tube," by Andrew V. H'ae'ffand Charles K. Birdsall, Serial No. 312,568, and filed on October 1,1952, now U. S. Patent No. 2,793,315. As described in the Haeif and Birdsall applications, an inductive wall for an electron stream may be obtained by utilizing a slow-wave material having a surface adjacent to the electron stream on which a resistive coating is deposited, while a highly conductive coating is deposited on the opposite surface. By slow-wave material is meant a material in which the velocity of propagation of an electromagnetic wave is substantially less than the velocity of light. I

Slow-wave characteristics may be obtained from several types'of' materials having suitable electromagnetic wave propagating characteristics; Ingeiie'ral, if V V represents the voltage (in kilovolt's) by which the stream electrons have been accelerated, then the necessary prerequisite for the slow-wave material is that the product ,u e be greater than or equal to I m xv wherein a is the permeability of the slow-wave material relative to that of free spaceand ER is the relative dielectric constant as measured for propagation transverse to thee-lectron stream-direction. In addition to the forego ing; it"is desirablethat the material used, have relativelylow'condu'ct-ive losses, particularly in the case of materials having-a'high dielectric constant. Materials satisfying these requirements may be divided into three groups} trio constant and alow permeability constant are the 'ti tanate bodies Which in'clude titanium dioxide, calcium titanate,-st rontium titanate and barium titanate, the rela: tive dielectric constant of these' materials being, .respec-. tively, of the order of 100, 150, 250 and 1000. Titana'te bodies' may b'e'p'roce'ssed so as to have the physical characteristics of a ceramic material which may be. readily produced in any desired shape as required.

Secondly, materials having a high permeability .constant at microwave frequencies include ferrites, the'gem eral chemical fo'r-mula for ferrities being R.Fe20 wherein.-

I R represents a nietal such as magnesium or copper; The permeability of ferrite" materials exhibits a: magnetic res.

onance at frequencies which are in the microwave range, thus, providing sufficiently high values of permeability to make their use practical as a slow-wave material. The advantage of using a material having a high permeability constant to decrease the velocity of propagation of an electromagnetic wave is that higher losses can be tolerated and a higher impedance maintained as compared to high dielectric constant materials.

A third way of providing a slow-wave material is by artificially producing the dielectric and magnetic characteristics by distributing small pieces of resonant metallic particles throughout a dielectric or magnetic material. As is commonly known, metallic particles are said to be resonant at a particular frequency When one dimension approximates one-half wavelength at the particular frequency.

At frequencies just less than the particular frequency at which resonance occurs, the metallic particles will exhibit inductive reactance. Methods of composing this type of material are well known in the art obviating the necessity of a more detailed description.

An inductive wall may be fabricated, for example, by using a material such as strontium titanate in a ceramic form shaped in a tubular configuration, the walls of which are preferably less than an odd multiple of onequarter wavelength thick and thicker than an even multiple of one-quarter wavelength as determined by the velocity of propagation and frequency of signal energy within the wall material. This thickness is necessary in order to present an inductive impedance to the electron stream. A resistive coating is then deposited on the inner surface contiguous to the electron stream and a highly conductive coating provided spaced from the electron stream by plating or evaporating a metal such as silver on the outer surface of the wall. A wall formed in this manner is analogous to a transmission line or waveguide, the conductive coating providing a shorting plane and the wall material providing a medium for the propagation of electromagnetic energy essentially unbounded in the transverse directions. The thickness of the Wall in wavelengths determines the impedance presented to the electron stream by the conductive coating or shorting plane on the outer surface of the wall, this impedance is in parallel with the resistance of the resistive coating.

The resistive coating deposited on the inner surface of the tubular element may consist of a layer of tin oxide formed by reacting tin chloride with a suitable agent on the surface material. A resistive coating of this type is much thinner than the skin depth representative of the penetration of microwave energy and, hence, exhibits a microwave frequency resistance that approximates that for direct currents. The resistivity of this coating is preferably as high as is compatible with collecting stray electrons for maintaining a uniform static potential throughout the length of the tubular element. Obviously this structure provides a purely inductive Wall only when infinite resistivity is approached, which limit will never be obtained due to the above requirements for maintaining a uniform static potential. In normal practice, suitable values of surface resistivity will be found to be of the order of 10,000 ohms per square centimeter. Alternatively, an inductive wall may be obtained by the use of an artificial dielectric material having resonance in the vicinity of the operating wavelength. Such a material can be made by imbedding numerous small pieces of metal in a dielectric material. In order to make an artificial dielectric material which responds to a broader bandwidth, the pieces of metal may be cut to different wavelengths within a specific band of frequencies and randomly scattered throughout the dielectric material.

Still another means for obtaining a distributed induct ance, is to use a thick wall made of ferrite material, as previously defined. Some ferrite materials have a permeability which goes through the equivalent of a parallel resonance in a rather narrow frequency band, making it necessary to use a particular ferrite material having a resonance in the region of a frequency spectrum whereamplification is desired. The width of this narrow frequency band would determine the bandpass characteristics of the tube and the particular ferrite material used. Since the permeability characteristic of this ferrite material goes through resonance at different frequencies, a broader response can be obtained by blending together two or more of the ferrite materials having resonance characteristics in the vicinity of the frequency range desired. The specific ferrite materials used are determined,

by the frequency and bandwidth to be used.

Referring now to Fig. 8 there is shown a wave-type amplifier tube similar to the tubes and associated circuits illustrated in Figs. 1 and 6 with the exception thatconductive tubular section 141, each section being one-- quarter plasma Wavelength long and disposed along the path of the electron stream in the direction of its flow in the order named.

Inductive wall sections 138, may be fabricated,

as previously described, by depositing resistive coatings 142, 143 having a surface resistivity of the order of 10,000 ohms per square centimeter on the inner surface of tubular elements 144, 145, respectively, and depositing highly conductive coatings 146, 147 on their outer surfaces. It is necessary that tubular elements 144, 145 be composed of a slow-wave material and preferably have a thickness less than one-quarter wavelength as determined by the frequency of the signal energy and the velocity of propagation of a wave at the signal frequency within the slow-Wave material.

An electron stream may be said to have spacial capacitance throughout the region through which it flows. An inductive wall positioned contiguous to the electron stream may produce two conditions, namely, it may either increase the capacitive reactance due to spacialcapacitance in the region occupied by the electron stream or secondly, it may completely cancel the spacial capacitance in the region thereby making it inductive. An increase in the capacitive reactance in the region occu pied by the electron stream has the effect of increasing the repulsion force between electrons due to the increased intensity of electric flux therebetween.

In the described embodiment of the present invention, the capacitive reactance of the region occupied by the electron stream is increased but not cancelled entirely by the utilization of an inductive wall such as provided by sections 138 and 140 positioned contiguous to the electron stream. The increase in the capacitive reactance has the effect of increasing the repelling forces between the electrons of the stream, thereby to increase effectively the reduction factor of the region. As previously mentioned, a large reduction factor region offers a high resistance to electron bunching and an aid to debunching. An increased repelling force between electrons of the stream obviously causes an increased resistance to electron bunching or an aid to debunching, hence, results in increasing the reduction factor of the region occupied by the electron stream.

Other phases of the operation of the described embodiment of the present invention are the same as for the foregoing tubes described in connection with Figs. 1 and 6. It is also apparent that an inductive wall section may be incorporated in the low noise level tube illustrated in Fig. 7 in place of helical element 116 of reduction factor section 115 if so desired.

15 What is claimed as new is:

1. A space charge wave discharge device comprising means for producing an electron stream, the electrons of said stream having perturbations corresponding to energy within a selected range of frequencies; means for directing said electron stream along a predetermined path whereby said perturbations produce a continual bunching and debunching of the electrons of said stream; first means maintained at a predetermined direct current potential and disposed along a first portion of said path for establishing a predetermined spatial capacitance throughout the region occupied by said stream along an interval commencing from a point of maximum electron bunching and extending in the direction of electron flow to the next succeeding point of minimum electron bunching; and second means effectively isolated from said first means as regards propagation of electro-magnetic waves and maintained also at said predetermined direct current potential and disposed along a second succeeding portion of said path for establishing a spatial capacitance different from said predetermined spatial capacitance throughout a region occupied by said stream along an interval commencing from a point of minimum electron bunching and extending in the direction of electron flow to the next succeeding point of maximum electron bunching thereby to eifect a change in the amplitude of said perturbations of the electrons of said stream.

2. A space charge wave discharge device according to claim 1 wherein the perturbations of the electrons of said stream are uniform and periodic and said spatial capacitance difierent from said predetermined spatial capacitance is more than said predetermined spatial capacitance thereby to increase the amplitude of said perturbations.

3. A space charge wave discharge device according to claim 2 wherein said means for establishing said different spatial capacitance that is more than said predetermined spatial capacitance comprises a conductive tubular member disposed concentrically about said path, the inner diameter of the bore of said tubular member being just sufiicient to accommodate said electron stream.

4. A space charge wave discharge device according to claim 2 wherein means for establishing said predetermined spatial capacitance comprises a conductive hollow cylindr'ical member disposed concentrically about said path, the inner diameter of said cylindrical member being substantially greater than the diameter of said electron stream.

5. A space charge wave discharge device according to claim 2 wherein means for establishing said predetermined spatial capacitance comprises a conductive helix disposed concentrically about said path, the inner diameter of said helix being adequate to accommodate said electron stream,

and said helix being capable of propagating an electromagnetic Wave, of a frequency corresponding to the periodicity of said perturbations at a velocity sufiiciently difierent from the velocity of said electron stream therethrough to preclude interaction between said stream.

6. A space charge wave discharge device according to claim 2 wherein means for establishing said predetermined spatial capacitance comprises a member having a wall disposed contiguous to said path and capable of presenting an inductive impedance to said electron stream.

7. A space charge wave discharge device according to claim 1 wherein said diiterent spatial capacitance is less than said predetermined spatial capacitance thereby to decrease the amplitude of said perturbations.

8. A space charge Wave discharge device comprising a means for producing an electron stream; means for density modulating said electron stream with modulating electromagnetic signals in a manner to produce in accordancc with said signals a continual bunching and debunching of the electrons in said stream with a predetermined plasma wavelength; a first cylindrical means disposed coaxially about said stream and maintained at a predetermined direct current potential and having a predetermined electromagnetic reduction factor and having an axial length of one quarter of said plasma wavelength and being longitudinally disposed with its input end about a point of maximum bunching of said electrons; a second cylindrical means maintained also at said predetermined potential and disposed coaxially about said stream down-stream from said first means and having a length of one quarter of said plasma Wavelength and being disposed with its input end about a point of minimum bunching of said electrons and having an electromagnetic reduction factor substantially less than said predetermined reduction factor and having an inner diameter so small as to just surround said electron stream and to substantially preclude propagation through said cylindrical means of electromagnetic waves whereby said bunching and debunching is materially aifected as said electron stream traverses said cylindrical means; and demodulating means disposed about said electron stream for deriving electromagnetic signals from said density modulated electron stream which are materially altered with respect to the amplitudes of said modulating electromagnetic signals.

References Cited in the file of this patent UNITED STATES PATENTS 2,403,795 Hahn July 9, 1946 2,653,271 Woodyard Sept. 22, 1953 2,692,351 Morton Oct. 19, 1954 2,762,948 Field Sept. 11, 1956 

